1. Field of the Invention
The present invention relates to a signal strength distribution establishing method and a wireless positioning system, and more particularly, to a signal strength distribution establishing method and a wireless positioning system capable of establishing a signal strength distribution map efficiently and correctly.
2. Description of the Prior Art
Position location (PL) techniques in wireless communication systems are used to estimate locations of terminal devices. Time of Arrival (TOA), Angle of Arrival (AOA) and Received Signal Strength (RSS) are common position location techniques. Time of Arrival-based technique firstly calculates distances between each of three base stations and a target by propagation velocity multiplying propagation time of received signals estimated by the three base stations respectively, takes each of the three base stations as a center of a circle and each of the distances as a radius for drawing circles, and thereby gets the target position by a meeting point of the three circles. Angle of Arrival-based technique determines source directions of received signals estimated by two base stations respectively, takes each base station position as a start point for forming a straight line, and thereby gets the target position by a meeting point of the two straight lines. RSS-based technique obtains distances between the target and the base stations according to received signal strength measured by mobile devices, and further derive the target position.
Since an indoor environment has complex furnishing and decoration, the radio signal propagation is not line of sight (or called Non-Line of Sight, NLOS) propagation, and a multipath effect is also quite obviously. The abovementioned TOA-based and AOA-based techniques are particularly affected by the multipath effect, and thereby easily cause errors during estimating the target position. On the other hand, variation of received signal strength is easy to estimate when the target moves, and thereby RSS-based technique are more suitable for an indoor position location system than TOA-based and AOA-based techniques.
RSS-based technique requires the received signal strength measured by mobile device, in addition, RSS-based technique needs to establish a signal strength distribution map (a.k.a. radio map). According the signal strength distribution map, the distances between the target and the base stations are derived. RSS-based technique then takes each base station as a center of a circle and each of the distances as a radius for drawing circles, and thereby determines the target position. In the prior art, to obtain the signal strength distribution map, the signal strength measured values from the plurality of base stations should be measured by mobile devices on different measuring locations in an indoor environment, and compute the signal strength values on the non-measuring locations by linear interpolation. In such a situation, an accuracy of the signal strength distribution map depends on how many measuring locations are deployed to measure the signal strengths. In general, more measuring locations yield a more accurate signal strength distribution map. In practice, an amount of measuring locations is limited. On the other hand, the signal strength values on the non-measuring locations are derived by linear interpolation. Linear interpolation has advantage of low computation complexity. However, linear interpolation over simplifies the variation of signal strength between two measuring locations, causing error between the computed signal strength distribution map and the actual signal strength distribution, such that the computed signal strength distribution map does not sufficiently represent the actual signal strength distribution. Therefore, how to establish the signal strength distribution map efficiently and correctly is a significant objective in the field.